1. Field of the Invention
The present invention relates to an optical disc device for reading information recorded on an optical disc medium such as a CD, a DVD, and a Blu-ray disc (registered trademark), a method of controlling the optical disc device, and an information storage medium having a program for controlling the optical disc device stored thereon.
2. Description of the Related Art
In recent years, various optical disc media are used as information recording media. In general, an optical disc medium is constructed of a plurality of layers such as a data recording layer on which the information is recorded and a protective layer for protecting the data recording layer. An optical disc device is used to read information recorded on such an optical disc medium. The optical disc device includes an optical pickup which irradiates the optical disc medium with light to detect reflected light from the optical disc medium.
When the optical disc device reads information recorded on the optical disc medium, an objective lens within the optical pickup needs to be brought into focus on a signal surface (surface of the data recording layer) of the optical disc medium. Accordingly, when reading information, the optical disc device performs, based on an output signal from the optical pickup, a focus detection operation of detecting an in-focus state in which the objective lens is in focus on the signal surface (see, for example, Japanese Patent No. 4001024).
By using a focus error signal (hereinbelow, referred to as FE signal) indicating a deviation of the focus of the objective lens with respect to the signal surface, the optical disc device can detect the in-focus state through an astigmatism method. Specifically, when the objective lens is gradually moved closer to the surface of the optical disc medium, the FE signal exhibits an S-shaped waveform around a timing at which the objective lens becomes in focus on the signal surface. By utilizing the above-mentioned waveform of the FE signal, the optical disc device detects the in-focus state. Hereinbelow, the position of the objective lens obtained when the objective lens is in focus on the signal surface is referred to as an in-focus position.
A specific example of a method of generating the FE signal is described. FIGS. 9A to 9C are diagrams each illustrating the shape of a spot S formed when a photodetector within the optical pickup is irradiated with reflected light from the optical disc medium. In FIGS. 9A to 9C, the spot 0 is indicated by an alternate long and short dash line. In this example, the photodetector includes four light-receiving elements Da to Dd arranged in a 2×2 matrix, and each of the light-receiving elements Da to Dd receives the reflected light from the optical disc medium, and outputs a signal indicating a light amount thereof. When the objective lens is in focus on the signal surface, the spot S becomes a circle as illustrated in FIG. 9A, and thus each of the light-receiving elements Da to Dd detects light having substantially the same light amount. However, when the focus of the objective lens becomes slightly deviated from the signal surface, the spot S becomes an ellipse, and, in addition, a major axis direction of the ellipse varies between a case where the objective lens is deviated in such a direction that the objective lens moves closer to the optical disc medium and a case where the objective lens is deviated in such a direction that the objective lens moves away from the optical disc medium. FIG. 9B illustrates the spot S obtained when the objective lens is closer to the optical disc medium compared to the in-focus position, whereas FIG. 9C illustrates the spot S obtained when the objective lens is farther from the optical disc medium compared to the in-focus position.
When La to Ld represent the levels of the output signals output from the light-receiving elements Da to Dd, respectively, an FE signal level Lfe is calculated, for example, as follows.Lfe=(La+Lc)−(Lb+Ld)The term (La+Lc) represents a total sum of amounts of light received by the light-receiving elements Da and Dc arranged along a diagonal line extending from the upper right to the lower left of a detection surface of the photodetector. This diagonal line corresponds to the major axis direction of the ellipse illustrated in FIG. 9B, which is formed by the spot S when the objective lens is closer to the optical disc medium compared to the in-focus position. Further, the term (Lb+Ld) represents a total sum of amounts of light received by the light-receiving elements Db and Dd arranged along a diagonal line intersecting the above-mentioned diagonal line. This diagonal line corresponds to the major axis direction of the ellipse illustrated in FIG. 9C, which is formed by the spot S when the objective lens is farther from the optical disc medium compared to the in-focus position. Accordingly, when the objective lens is gradually moved closer to the optical disc medium, the spot S first takes the shape illustrated in FIG. 9C before the in-focus position, and the FE signal exhibits a negative peak. After that, the spot S becomes the circle illustrated in FIG. 9A at the in-focus position, and the magnitude of the FE signal at that time becomes 0. When the objective lens is further moved closer to the optical disc medium, the spot S takes the shape illustrated in FIG. 9B, and the FE signal in this case exhibits a positive peak. FIG. 10 illustrates an S-shaped waveform exhibited by the FE signal in the vicinity of the in-focus position when the above-mentioned control is executed. The optical disc device can identify the in-focus position of the objective lens by detecting, while bringing the objective lens closer to the optical disc medium, a timing at which the FE signal becomes 0 after exhibiting the negative peak once, or by detecting, while moving the objective lens away from the optical disc medium, a timing at which the FE signal becomes 0 after exhibiting the positive peak. Once the in-focus position is identified, the optical disc device performs feedback control (focus servo control) of adjusting the position of the objective lens so as to maintain this level of the FE signal, with the result that it is possible to maintain a state in which the objective lens is in focus on the signal surface of the optical disc medium. The reading of information from the optical disc medium is performed when the focus servo control is in execution.
In the above-mentioned optical disc device of the related art example, if the position of the spot S illustrated in FIG. 9A is deviated from the center position of the photodetector, it becomes difficult to detect the in-focus position accurately. This is because if the position of the spot S is deviated from the center position, the magnitude of the FE signal does not become 0 and, instead, becomes deviated to a positive or negative side even when the spot S is a circle. Such a deviation of the FE signal is caused by various factors. For example, if the mounting position of the photodetector is deviated from the optical axis of the reflected light at the time of manufacturing the optical disc device, there occurs a deviation in the FE signal as well. Further, while the optical disc device is in use, in addition to the above-mentioned focus servo control, there is also performed tracking servo control in which the objective lens is moved along a radial direction of the optical disc medium so that the objective lens follows a track provided to the signal surface of the optical disc medium. In the tracking servo control, the objective lens is moved along a direction parallel to the surface of the optical disc medium in a relative manner with respect to the optical axis of a laser beam. Accordingly, due to the movement of the objective lens resulting from the above-mentioned tracking servo control, the relative position of the reflected light with respect to the photodetector also moves. Such relative movement of the objective lens causes the S-shaped waveform which appears in the FE signal to become asymmetric with respect to the in-focus position, which may have an adverse effect on accuracy in focus detection.